DE69803669T2 - ISOCYANATE COMPOSITIONS FOR FOAMED POLYURETHANE FOAMS - Google Patents

Description

The present invention relates to processes for producing rigid polyurethane foams and to reaction systems for use therein. More specifically, the present invention is directed to processes for producing rigid polyurethane foams using a specific polyisocyanate composition, an isocyanate-reactive composition and hydrofluorocarbon or hydrogen-carbon blowing agents.

Rigid polyurethane foams have many well-known uses, such as in construction materials and for thermal insulation. Such foams are known for exhibiting superior structural properties, excellent initial and long-term thermal insulation properties and good flame retardant properties.

Rigid polyurethane foams have traditionally been produced by reacting suitable polyisocyanate and isocyanate-reactive compositions in the presence of a suitable blowing agent. Regarding the blowing agents, chlorofluorocarbons (CFCs) such as CFC-11 (CC13F) and CFC-12 (CC12F2) have been used most intensively because they have been shown to produce foams with good thermal insulation properties, low flammability and excellent dimensional stability. Despite these advantages, CFCs have fallen out of favor because they have been associated with ozone depletion in the Earth's atmosphere and with a possible global warming potential. Accordingly, the use of CFCs has been strictly restricted.

Hydrogen chlorofluorocarbons (HCFCs) such as HCFC-141b (CC12FCH3) and HCFC-22 (CHClF2) have been widely used as an interim solution. However, HCFCs have also been shown to cause similar ozone depletion in the Earth's atmosphere and, accordingly, their use has been closely monitored. In fact, the widespread manufacture and use of HCFCs is scheduled to end soon.

Therefore, there was a need to develop processes for producing rigid polyurethane foams which use blowing agents without ozone-depleting potential and which still provide foams with excellent thermal insulation properties and excellent dimensional stability.

One class of materials that have been investigated as such blowing agents includes various hydrogen carbons such as n-pentane, n-butane and cyclopentane. The use of such materials is well known and disclosed, for example, in US Patent Nos. US-A-5,096,933, US-A-5,444,101, US-A-5,182,309, US-A-5,367,000 and US-A-5,387,618. However, it has been found that these known processes for producing foams with such blowing agents and the reaction systems used in such processes do not produce rigid polyurethane foams with commercially attractive physical properties at sufficiently low densities to enable their use. In short, the properties associated with such hydrogen carbon foams are generally inferior to those of CFC and HCFC foams.

Attention has also turned to the use of hydrogen fluorides (HFCs), including 1,1,1,3,3-pentafluoropropane (HFC 24Sfa), 1,1,1,3,3-pentafluorobutane (HFC 36Smfc), 1,1,1,2-tetrafluoroethane (HFC 134a) and 1,1-difluoroethane (HFC 152a). The use of such materials as blowing agents for rigid polyurethane foams is disclosed, for example, in U.S. Patent Nos. US-A-5,496,866, US-A-5,461,084; US-A- 4,997,706, US-A-5,430,071 and US-A-5,444,101. However, attempts to produce rigid foams using hydrogen carbons with such materials have generally not produced foams with structural, thermal and heat properties comparable to those obtained using CFC-11 as the blowing agent.

The majority of attempts to solve this problem have focused on blending different hydrogen fluorocarbons, hydrogen carbons, or blending hydrogen carbons with hydrogen fluorocarbons and/or other propellants. Such attempts have had limited success.

Accordingly, there remained a need for a process for producing a rigid polyurethane foam which uses hydrogen fluorocarbon or hydrogen carbon blowing agents and which provides foams with excellent physical properties.

This object is achieved by the present invention, which uses polymeric polyisocyanates of a specific composition in the process for producing rigid polyurethane foams with hydrogen fluorocarbon or hydrogen carbon blowing agents. The present invention provides foams with improved physical and thermal insulation properties.

The present invention is directed to a process for producing rigid polyurethane foams, comprising the reaction of:

(1) a polyphenylene polymethylene diisocyanate composition;

(2) an isocyanate-reactive composition containing a plurality of isocyanate-reactive groups, which is useful for the production of rigid polyurethane foams or urethane-modified rigid polyisocyanurate foams;

(3) a hydrogen fluorocarbon or hydrogen carbon propellant;

(4) optionally by water or other carbon dioxide-evolving compounds and

wherein the polyphenylene polymethylene polyisocyanate comprises the following:

(a) 15 to 42% by mass of diphenylmethane diisocyanate, based on 100% of the polyisocyanate component (1);

(b) 3-ring oligomers of polyphenylenepolymethylenepolyisocyanate (hereinafter referred to as triisocyanate) in an amount such that the ratio of diisocyanate to triisocyanate is between 0.2 and 1.8; and

(c) a residue of higher homologues of polyphenylenepolymethylene polyisocyanate.

The present invention is further directed to a reaction system useful for producing rigid polyurethane foams, comprising:

(1) a polyphenylene polymethylene polyisocyanate composition;

(2) an isocyanate-reactive composition containing a plurality of isocyanate-reactive groups, which are useful for the production of rigid polyurethane foams or urethane-modified polyisocyanurate foams;

(3) a hydrogen fluorocarbon or hydrocarbon propellant;

(4) optionally water or other carbon dioxide-evolving compounds, and

wherein the polyphenylene polymethylene polyisocyanate comprises the following:

(a) 15 to 42% by mass of diphenylmethane diisocyanate, based on 100% of the polyisocyanate component (1);

(b) 3-ring oligomers of polyphenylenepolymethylenepolyisocyanate (hereinafter referred to as triisocyanate) in an amount such that the ratio of diisocyanate to triisocyanate is between 0.2 and 1.8; and

(c) a residue of higher homologues of polyphenylenepolymethylene polyisocyanate.

The polyphenylenepolymethylene polyisocyanates used in the present invention are those of the formula I:

The 3-ring oligomers of component 1(b) are those represented by formula I, where n = 1. The higher homologues of component 1(c) are those represented by formula I, where n > 1.

The polyphenylene polymethylene polyisocyanate composition (1) used in the present invention comprises 15 to 42 mass%, preferably 20 to 40 mass%, and more preferably 24 to 38 mass% of diphenylmethane diisocyanates based on 100% of the polyisocyanate component. Diphenylmethane diisocyanate in the form of its 2,2'-, 2,4'- and 4,4'-isomers and mixtures thereof can be used in the present invention. Any variation of the 2,2'-, 2,4'- and 4,4'-isomers can be used.

The polyphenylenepolymethylenepolyisocyanate composition (1) further comprises the triisocyanate component in an amount such that the ratio of diisocyanate to triisocyanate is between 0.2 to 1.8, and preferably between 0.33 to 1.8. Thus, the actual triisocyanate content is determined based on the amount of diphenylmethane diisocyanate in the polyphenylenepolymethylene composition (1) using the ratio given above. The amount is based on a mass percent based on 100 mass percent of the total polyisocyanate composition.

For purposes of clarification, if the amount of diphenylmethane diisocyanate in a given polyphenylene polymethylene polyisocyanate composition is 30 percent and the ratio of diisocyanate to triisocyanate is 1.5, then the amount of triisocyanate that the polyphenylene polymethylene polyisocyanate composition must comprise would be 20 percent by weight based on 100 percent by weight of the total composition. The term "triisocyanate" as used herein means all isomers of the 3-ring oligomers of polyphenylene polymethylene polyisocyanate (i.e., n = 1 in Formula I) containing three phenyl, two methyl and three isocyanate groups. Seven possible isomers of triisocyanate are described in "Chemistry and Technology of Isocyanates" by Henri Ulrich, John Wiley & Sons Inc., p. 388 (1996).

The remainder of the polyphenylenepolymethylenepolyisocyanate composition comprises higher homologues of the polyphenylenepolymethylenepolyisocyanate. The higher homologues include all higher than tri-, i.e. tetraisocyanate, heptaisocyanate, hexaisocyanate, etc. (i.e. n > 1 in structure 1). Suitable higher homologues are in "The Polyurethanes Book", edited by George Woods, John Wiley & Sons Publisher (1987). The amount of higher homologues contained in the polyphenylene polymethylene polyisocyanate composition is generally from 10 to 77 percent and preferably from 19 to 69 percent, based on 100 percent by weight of the total composition.

The higher homolog component (c) may further comprise higher functionality isocyanates modified with various groups including ester groups, urea groups, biuret groups, allophanate groups, carbodiimide groups, isocyanurate groups, urethdione groups and urethane groups. Such modified isocyanates and methods for their preparation are known in the art.

The polyphenylene polymethylene polyisocyanate composition (1) is used in an amount of 35 to 70 of the total reaction system.

The polyphenylenepolymethylenepolyisocyanate composition (1) can be prepared by methods known to those skilled in the art. Suitable methods are disclosed, for example, in "Chemistry and Technology of Isocyanates", Henri Ulrich, John Wiley & Sons Inc. (1996). In general, the polyphenylenepolymethylenepolyisocyanate compositions are prepared by the reaction of aniline with formaldehyde under acidic conditions to produce amines. This is followed by phosgenation and thermal cleavage of the resulting material in a mixture of isocyanate homologues. The amount of diphenylmethane diisocyanate, triisocyanate and higher homologues in the composition can be adjusted by adjusting the ratio of aniline to formaldehyde and/or the Reaction conditions can be manipulated. For example, a higher aniline to formaldehyde ratio results in a polyphenylenepolymethylenepolyamine containing higher amounts of the diphenylmethanediamine component and the triamine component and correspondingly lower yield of the higher homolog component. Therefore, phosgenation and thermal cleavage of the resulting polyphenylenepolymethylenepolyamine results in a polyphenylenepolymethylenepolyisocyanate product containing higher amounts of the diphenylmethane diisocyanate and the triisocyanate and lower amounts of the higher homolog isocyanate. In addition, the composition of the polyphenylenepolymethylenepolyisocyanate component can also be controlled by partial fractionation to separate diphenylmethane diisocyanate along with a variety of isocyanate-reactive reaction routes.

The isocyanate-reactive compositions (2) useful in the present invention include all those known to those skilled in the art as useful for producing rigid polyurethane foams. Examples of suitable isocyanate-reactive compositions having multiple isocyanate-reactive groups include polyether polyols, polyester polyols and mixtures thereof having an average hydroxyl number of from 20 to 1000 and preferably from 50 to 700 KOH/g and a hydroxyl functionality of from 2 to 8 and preferably from 2 to 6. Other isocyanate-reactive materials which can be used in the present invention include hydrogen-determined polythioethers, polyamides, polyesteramides, polycarbonates, polyacetals, polyolefins, polysiloxanes and polymer polyols.

Suitable polyether polyols include reaction products of alkylene oxides, e.g. ethylene oxide and/or propylene oxide with initiators containing 2 to 8 active hydrogen atoms per molecule. Suitable initiators include polyols, e.g. diethylene glycol, glycerin, trimethylolpropane, triethanolamine, pentaerythritol, sorbitol, methyl glucoside, mannitol and sucrose; polyamines, e.g. ethylenediamine, toluenediamine, diaminodiphenylmethane and polymethylenepolyphenylenepolyamines; amino alcohols, e.g. ethanolamine and diethanolamine; and mixtures thereof. Preferred initiators include polyols and polyamines.

Suitable polyester polyols include those prepared by reacting a carboxylic acid and/or a derivative thereof or a polycarboxylic acid anhydride with a polyhydric alcohol. The polycarboxylic acids may be any known aliphatic, cycloaliphatic, aromatic and/or heterocyclic polycarboxylic acids and may be substituted (e.g. with halogen atoms) and/or unsubstituted. Examples of suitable polycarboxylic acids and anhydrides include oxalic acid, malonic acid, glutaric acid, pimelic acid, succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, terephthalic acid, trimellitic acid, trimelitic anhydride, pyromellitic dianhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, glutaric anhydride, maleic acid, maleic anhydride, fumaric acid and dimeric and trimeric fatty acids such as those of oleic acid, which may be present in admixture with monomeric fatty acids. Simple esters of polycarboxylic acids can be used, as can dimethyl terephthalate, terephthalic bisglycol and extracts thereof. While the aromatic polyester polyols from While the phthalic acid, phthalic anhydride, terephthalic acid, dimethyl terephthalate, polyethylene terephthalate and the like can be produced from essentially pure starting materials as listed above, more complex ingredients can advantageously be used, such as the effluents, rejects or scrap residues from phthalic acid, phthalic anhydride, terephthalic acid, dimethyl terephthalate and polyethylene terephthalate production and the like.

The polyhydric alcohols suitable for preparing polyester polyols can be aliphatic, cycloaliphatic, aromatic and/or heterocyclic alcohols. The polyhydric alcohols can optionally include substituents which are inert in the reaction, e.g. chlorine and bromine substituents and/or can be unsaturated. Suitable amino alcohols such as monoethanolamine, diethanolamine or the like can also be used. Examples of suitable polyhydric alcohols include ethylene glycol, propylene glycol, polyoxyalkylene glycols (such as diethylene glycol, polyethylene glycol, dipropylene glycol and polypropylene glycol), glycerin and trimethylolpropane.

The isocyanate-reactive material is used in an amount of 20% to 70% and preferably from 30% to 60% of the total reaction system.

The present process further comprises reacting the polyphenylene polymethylene polyisocyanate composition (1) and the isocyanate-reactive composition (2) with one or more hydrogen fluorocarbon or hydrogen carbon blowing agents which are vaporizable under the foam forming conditions.

The hydrogen fluorocarbon propellants useful in the present invention include: 1,1,1,3,3-pentafluoropropane (HFC-24Sfa); 1,1,1,3,3-pentafluorobutane (HFC 365mfc); 1,1,1,4,4,4-hexafluorobutane (HFC 356mff); 1,1-difluoroethane (HFC 152a); 1,1,1,2-tetrafluoroethane (HFC 134a) and mixtures thereof. Preferred hydrogen fluorocarbons include: 1,1,1,3,3-pentafluoropropane; 1,1,1,3,3-pentafluorobutane and 1,1,1,2-tetrafluoroethane. Suitable hydrogen carbons include butane, isobutane, isopentane, n-pentane, cyclopentane, 1-pentene, n-hexane, isohexane, 1-hexene, n-heptane, isoheptane and mixtures thereof. Preferably, the hydrogen carbon propellant is isopentane, n-pentane, cyclopentane and mixtures thereof. The most preferred hydrogen carbon propellant for use in the present invention is a mixture of isopentane to n-pentane in a ratio of 80:20 to 99:1 parts by weight.

The hydrogen fluorocarbon blowing agent should be used in an amount of 2% to 20% and preferably 4% to 15% of the total reaction system.

The hydrogen carbon blowing agent should be used in an amount of 2% to 20% and preferably 4% to 15% of the total reaction system.

Other physical blowing agents may also be used in the present process in combination with the hydrogen carbon blowing agents. Suitable blowing agents include: 1,1,1,3,3- pentafluoropropane (HFC-245fa); 1,1,1,2-tetrafluoroethane (HFC-134a); 1,1-difluoroethane (HFC-152a); difluoromethane (HFC-32); chlorodifluoromethane (HCFC-22) and 2-chloropropane. In use, these blowing agents may be incorporated into the isocyanate-reactive component, the isocyanate component and/or as a separate stream to the reaction system.

Volatile non-hydrofluorocarbons such as 2-chloropropane, isopentane and cyclopentane can also be used in the present process in combination with the hydrofluorocarbon blowing agents. When used, the blowing agents can be mixed into the isocyanate-reactive component, the isocyanate component and/or as a separate stream to the reaction system.

The present process may further optionally comprise reacting the polyphenylene polymethylene polyisocyanate, the isocyanate-reactive composition and the hydrogen fluorocarbon or hydrogen carbon blowing agent in the presence of water in an amount of from 0.1% to 5%, and preferably from 0.2% to 4% of the total reaction system. Water reacts to produce carbon dioxide to act as an additional blowing agent. Other carbon dioxide generating compounds may further be employed in place of or in addition to water. Such compounds include carboxylic acids and cyclic amines.

The reaction system may further comprise one or more auxiliaries or additives if required for one or more particular purposes. Suitable auxiliaries and additives include crosslinking agents such as triethanolamine and glycerin; foam stabilizing agents or surfactants such as siloxane/oxyalkylene copolymers and oxyethylene/oxyalkylene copolymers; catalysts such as tertiary amines (e.g. dimethylcyclohexylamine, pentamethyldiethylenetriamine, 2,4,6-tris(dimethylaminomethyl)phenol and triethylenediamine), organometallic compounds (e.g. potassium octoate, potassium acetate, dibutyltin dilaurate), quaternary ammonium salts (e.g. 2-hydroxypropyltrimethylammonium formate) and N-substituted triazines (N,N'N"-dimethylaminopropylhexahydrotriazine); flame retardants such as organophosphorus compounds such as organic phosphates, phosphites, phosphonates, polyphosphates, polyphosphites, polyphosphonates, ammonium polyphosphate (e.g. triethyl phosphate, diethyl ethyl phosphonate, tris(2-chloropropyl)phosphate)) and halogenated compounds (such as tetrabromophthalester, chlorinated paraffins); viscosity reducing agents such as propylene carbonate and 1-methyl-2-pyrrolidinone; opacifiers in the infrared range such as carbon black, titanium dioxide and metal flakes; cell size reducing compounds such as inert, insoluble fluorinated compounds and perfluorinated compounds; reinforcing agents such as glass fibers and ground foam scrap; mold release agents such as zinc stearate; antioxidants such as butylated hydroxytoluene; and pigments such as azo/diazo dyes and phthalocyanines. The amount of such auxiliary materials or additives is generally between 0.1 to 20% by weight, preferably between 0.3 to 15% by weight, and most preferably between 0.5 to 10% by weight, based on 100% by weight of the total foam formulation.

In operating the process according to the invention for producing rigid foams, the known single-stage, prepolymer or semi-prepolymer technique can be used together with conventional mixing methods such as impact mixing. The rigid foam can be in the form of a slabstock foam, molded foam, cavity foam, sprayed foam, blow-molded foam, or laminated with other materials such as paper, metal, plastics, or wood boards. See, for example, Saunders and Frisch, Polyurethanes Chemistry and Technology, Part II, Interscience Publishers, New York (1962), and the references cited for various polyurethane manufacturing processes.

The present invention further includes rigid polyurethane foams produced by the processes disclosed above.

The present invention will now be illustrated with reference to the following specific non-limiting examples.

Examples

Unless otherwise noted in the examples described below, all temperatures are expressed in degrees Celsius and the amounts of all formulation components are expressed in parts by weight.

The following materials are used and referred to in the examples.

Stepanpol® PS-2352 is an aromatic polyester polyol, available from Stepan Co., which comprises a phthalic anhydride/glycol based polyol with a hydroxyl value of 240 KOH/g and a viscosity at 25ºC of 3,000 cPs.

TCPP is tri(beta-chloropropyl) phosphate, available from Great Lakes Chemical Corporation.

Pelron® 9540A is potassium octoate in diethylene glycol, available from Pelron Corp.

Polycat® 8 is dimethylcyclohexylamine, available from Air Products Corp.

Tegostab® B8466 is a silicone surfactant available from Goldschmidt Corporation.

Borger Isopentane is an isopentane product containing 97.5% isopentane and 2.5% n-pentane, available from Phillips Petroleum Company.

Hydrogen Fluorocarbon HFC 245fa (under pressure), available from AlliedSignal.

Polyisocyanate A contained 32% diphenylmethane diisocyanates, had a diisocyanate to triisocyanate ratio of 1.2 (providing the triisocyanate in an amount of 26.7%); and 41.3% higher homologues. Isocyanate B had a diphenylmethane diisocyanate content of 44%; a diisocyanate to triisocyanate ratio of 1.8 (providing 24.4% triphenylmethane triisocyanate); and 31.6% higher homologues. Both isocyanates A and B had an NCO content of 31%.

example 1

A polyol blend was prepared by mixing 100 parts Stepanpol PS2352 with 14 parts TCPP, 3 parts Pelron 9540A, 0.6 parts Polycat 8, 2.65 parts Tegostab B8466 and 1.3 parts water in a high speed mixer at room temperature.

Rigid foams were prepared from the formulations given in Table 1 below. The polyol blend was added to the "B-side" tank of an Edge Sweets high pressure impingement mixing disperser. An appropriate amount of isopentane, based on the compositions given in Table 1, was then added to the "B-side" and mixed vigorously using an air mixer attached to the tank. The isocyanate was then added to the "A-side" tank attached to the disperser.

The device parameters were set as follows:

The ingredients for foam production were shot from the dispersing machine into Lily Cups No. 10 and the reactivity was measured on the freely used foam.

The structural properties were measured on core samples taken from 7" x 7" x 15" foams prepared by dispersing the foam ingredients in a suitable cardboard box.

The foam core density was measured according to ASTM D1622: The dimensional stability at high Temperature was measured according to ASTM D2126. Compressive strength was measured parallel and perpendicular to the direction of foam expansion according to ASTM D1621 Procedure A. Thermal properties of the foams were measured according to ASTM C518 on core foam taken from 2" x 14" x 14" blocks. Fire performance was tested according to ASTM D3014 to measure Butler Chimney mass retention. Table 1

As is clear from the data presented in Table 1, Foam 1 made with the inventive isocyanate A is a rigid polyurethane foam which has superior structural and thermal properties and superior fire performance compared to Foam 2. Foam 2 was made with Isocyanate B, which is outside the scope of the present invention.

Foams 3 and 4 were made with densities typical of foams expanded with CFC. As shown in Table 1, foam 3 made with isocyanate A of the present invention has superior structural and thermal properties and fire performance compared to foam 4. Foam 4 was made with isocyanate B, which is outside the scope of the present invention.

Furthermore, Foam 3 (according to the present invention) can be compared to Foam 2. The dimensional stability and Butler-Chimney mass retention are almost identical for the two foams. Also, the compressive strength along with the initial and aged k-factors of Foam 3 are superior to those of Foam 2. Accordingly, the data show that foams made with a polyisocyanate composition (Isocyanate A) according to the present invention have better performance properties at lower densities than those foams made with conventional isocyanates at higher densities.

Example 2

A polyol blend was prepared by mixing 100 parts Stepanpol PS2352 with 4.5 parts Pelron 9540A, 1.0 parts Polycat 8, 2.0 parts Tegostab B8466 and 0.3 parts water in a high speed mixer at room temperature.

Rigid foams were prepared from the formulations given in Table 1 below. The polyol blend was added to the "B-side" tank of an Edge Sweets high pressure impingement mixing disperser. An appropriate amount of isopentane, based on the compositions given in Table 1, was then added to the "B-side" and mixed vigorously using an air mixer attached to the tank. The isocyanate was then added to the "A-side" tank attached to the disperser.

The device parameters were set as follows:

The ingredients for foam production were shot from the dispersing machine into Lily Cups No. 10 and the reactivity was measured on the freely used foam.

The structural properties were measured on core samples taken from 7" x 7" x 15" foams prepared by dispersing the foam ingredients in a suitable cardboard box.

Foam core density was measured according to ASTM D1622. High temperature dimensional stability was measured according to ASTM D2126. Compressive strength was measured parallel and perpendicular to the direction of foam expansion according to ASTM D1621 Procedure A. Thermal properties of the foams were measured according to ASTM DC518 on core foam taken from 2" x 14" x 14" blocks. Fire performance was tested according to ASTM D3014 to measure Butler Chimney mass retention. Table 1

As is clear from the data presented in Table 1, Foam 1 made with the inventive isocyanate A is a rigid polyurethane foam which has superior structural and thermal properties and superior fire performance compared to Foam 2. Foam 2 was made with Isocyanate B, which is outside the scope of the present invention.

Foams 3 and 4 were made with densities typical of foams expanded with CFC. As shown in Table 1, foam 3 made with isocyanate A of the present invention has superior structural and thermal properties and fire performance compared to foam 4. Foam 4 was made with isocyanate B, which is outside the scope of the present invention.

Furthermore, Foam 3 (according to the present invention) can be compared to Foam 2. The dimensional stability value is almost identical for the two foams. Also, the compressive strength, along with the initial and aged K-factors of Foam 3, is superior to that of Foam 2. Accordingly, the data show that foams made with a polyisocyanate composition according to the present invention have better performance properties at lower densities than those foams made with conventional isocyanates at higher densities.

Claims (9)

1. A process for producing a rigid polyurethane foam, the process comprising reacting a polyisocyanate composition with an isocyanate-reactive composition in the presence of a hydrogen fluorocarbon blowing agent, characterized in that the polyisocyanate composition comprises (a) 15 to 42 mass % of diphenylmethane diisocyanate, (b) three-ring oligomers of polyphenylene polymethylene polyisocyanate in an amount such that the ratio of (a) to (b) is from 0.2 to 1.8, and (c) higher homologues of polyphenylene polymethylene polyisocyanate. 2. The method of claim 1, wherein the amount of hydrofluorocarbon is from 2% to 20% by mass of the composition. 3. The method of claim 2, wherein the amount of hydrofluorocarbon is from 4% to 15% by mass of the composition. 4. The process of claim 1, wherein the hydrogen fluorocarbon is selected from the group consisting of 1,1,1,3,3-pentafluoropropane (HFC-245fa); 1,1,1,3,3-pentafluorobutane (HFC 365mfc); 1,1,1,4,4,4-hexafluorobutane (HFC 356mff); 1,1-difluoroethane (HFC 152a); 1,1,1,2-tetrafluoroethane (HFC 134a) and mixtures thereof. 5. A process for producing a rigid polyurethane foam, the process comprising reacting a polyisocyanate composition with an isocyanate-reactive composition in the presence of a hydrogen-carbon blowing agent, characterized in that the polyisocyanate composition comprises (a) 15 to 42 mass % of diphenylmethane diisocyanate, (b) three-ring oligomers of polyphenylene polymethylene polyisocyanate in an amount such that the ratio of (a) to (b) is from 0.2 to 1.8, and (c) higher homologues of polyphenylene polymethylene polyisocyanate. 6. The method of claim 5, wherein the amount of hydrocarbon is from 2% to 20% by mass of the composition. 7. The method of claim 6, wherein the amount of hydrocarbon is from 4% to 15% by mass of the composition. 8. The process of claim 5, wherein the hydrocarbon is selected from the group consisting of butane, isobutane, isopentane, n-pentane, cyclopentane, 1-pentene, n-hexane, isohexane, 1-hexene, n-heptane, isoheptane and mixtures thereof. 9. The process of claim 5, wherein the hydrocarbon is present as a mixture of isopentane and n-pentane in a ratio of 80:20 to 99:1 parts by weight.